1. Field of the Invention
The present invention relates to cold trap systems and specifically to an efficient and easily cleaned high pressure cold trap system.
2. Prior Art
Cold traps are used to remove gases having a relatively high temperature of solidification, or sometimes liquefaction, from a gas with a lower temperature of solidification. They can also be used to remove particulate matter from the gas. Cold traps are usually used to remove gaseous or particulate contaminants from a vapor stream. The condensate, which may comprise molecules of a higher vapor pressure than the dry gas produced, is typically disposed of. Cold traps are often used to remove water from a compressed air supply, or to remove hydrocarbons from a vacuum system.
A cold trap provides a cold surface which the contaminant is likely to encounter. Molecules are immobilized by freezing onto or by adsorption into the cold surface. The mean free path of molecular flow through a cold trap is determined by the trap's operating pressure. The higher the pressure the shorter the mean free path. To insure that all of the molecules eventually collide with a cold surface, the flow paths must be more circuitous at higher pressures. Therefore, cold traps are designed for and are available for different operating pressure ranges.
Cold traps are well known for the submillimeter vacuum pressure range of 10.sup.-2 to 10.sup.-9 mm Hg. At these low pressures the mean free path of a molecule is relatively long, 3 mm to 3.times.10.sup.7 cm. Low pressure traps typically employ chevrons or simple baffle arrangements that prevent molecules from traveling in a straight line through the trap. However, these traps are ineffective at pressures above 0.5 mm. At higher pressures, molecules must encounter a cold surface within a short distance to have an adequate probability of being caught. Otherwise molecules block each other in their passage through the trap.
High pressure traps are also well known for removing water vapor from compressed air streams, or for removing other vapors which, when condensed, have a low enough viscosity to drain from the trap. However, these traps are not suitable for condensing gummy contaminant substances which can be formed. For example, polyurethane resin has components such as Toluene Diisocyanate (TDI) and Methylene Di-paraphenylene Isocyanate (MDI). When a vacuum pump is used to draw vapor from a tank of liquid polyurethane resin, the TDI's and MDI's can react with the pump's lubricating oil to produce gummy contaminants. TDI and MDI vapors can also react with moisture, and to some extent other impurities in a vapor stream, to form a gum. Prior art cold traps, used to remove such gummy contaminants, must be periodically scrubbed with strong hot solvents to remove the residue.
There is a need for a cold trap to operate at pressures ranging from 0.5 mm to 760 mm, at -40.degree. C., and which can be used to remove vapors such as TDI's and MDI's, mists and gummy substances from a vapor stream. The trap should be quickly heatable to defrost the condensate, should allow melted material to drain out, and should be easy to disassemble, clean and reassemble.